If I NFS-export those file systems and run the exact same build.sh on my
dual-core Athlon 64 X2 the build takes twice as long as normal but runs to
completion without errors.

So seems to definitely be related to load. Either that or I'm getting bitten
by the "more than 4GB RAM bounce buffer" problem, but the symptoms of that
are usually less severe - just random failures of gcc or other build tool.

Is there an easy way to disable the extra RAM (other than opening up the box
and removing DIMMs)?

Interesting. I added "options PHYSMEM_MAX_ADDR=4096" to the 8GB quad-

core box and it now survives a 'build.sh -j8 release' (although it is
about 20% to 25% slower that it used to be with the full 8GB enabled.

Just curious - even though I set the option above to 4GB, it seems to be
using only 3GB: